# Theory of relativity

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$E=mc^2$

The theory of relativity was developed by Albert Einstein in the early 1900s. There are two theories of relativity. The first is special relativity and the second is general relativity. Both are based on the principle of relativity, which was created by Galileo Galilei in the 1600s.

## Special relativity

Special relativity says that every person has their own time. One person's clock says something different from another person's clock. The reason a person's time can be different from another's is because of time dilation, which can be thought of more easily by the twin paradox.

### Twin paradox

The twin paradox shows how time changes because of speed. The faster someone or something moves or the more mass they have, then the slower time passes for them. This is shown by a pair of twins. Imagine that the first twin moves at a fast speed, near the speed of light for example, while the other twin stays in one general location. When the first twin returns from his trip, he will be younger than the second twin. Thus time and speed are related

## References

• Hawking, Stephen. The Universe in a Nutshell. Bantam Books, 2001.